When laying a pipeline at sea one of two methods is commonly used: either the “S” laying method or the “J” laying method. The two methods are named in accordance with the general shape adopted by the pipeline during laying.
In “S” laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. The tension in the pipeline is often accommodated by a series of track tensioners mounted along the pipe-laying path, on the vessel. The track tensioners support the weight of the pipeline and control passage of the pipeline.
In “J” laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. “J” laying often involves moving a new pipeline section from a substantially horizontal position (along the deck of the vessel) into a vertical position to align with a J lay tower mounted on the vessel. A lower end of the new pipeline section is welded to the upper end of the pipeline, which is held in place, suspended from the vessel, by a fixed clamp, located towards the bottom of the J lay tower. Once the new section of pipeline has been added to the existing pipeline, the fixed clamp is released and the pipeline lowered down the J lay tower. During such lowering the tension in the pipeline may be accommodated by a travelling block or track tensioners. The upper end of the newly lengthened pipeline (i.e. the upper end of the new section of pipeline) is then clamped by the fixed clamp, the travelling block, if used, is returned to its original position towards the top of the J lay tower and the process repeated.
As interest in laying pipeline in deep water has increased, so “J” laying has become more attractive because the pipeline naturally adopts a vertical or near-vertical orientation far from the seabed. “J” laying is not, however, preferred in shallower water where the natural path of the pipeline is only ever inclined at a shallow angle as it passes to the seabed and “S” laying is therefore advantageous.
“S” laying can be employed in deep water provided the pipeline being laid can be supported from the vessel until it has reached a relatively steep inclination to the horizontal. That, however, requires a considerable length of support, because the radius of bending to which the pipeline can be subjected is limited, especially in the case of large diameter pipelines.
WO 2007/000609 A2 discloses a pipe-laying vessel with S-lay apparatus. The S-lay apparatus includes a stinger for supporting the pipeline in an arcuate path so that a departure axis of the pipeline is more vertical than horizontal. The S-lay apparatus of the vessel comprises a fixed clamp and a travelling clamp mounted on the deck of the vessel. These are used to feed the pipeline along the S-lay pipe-laying path. The vessel can also comprise J-lay apparatus mounted above the lower end of the stinger, substantially aligned with the departure axis of the pipeline so as to facilitate handing over the suspended pipeline between the S-lay apparatus and the J-lay apparatus such that different parts of the same pipeline can be fabricated and laid by the different types of apparatus.
WO 2006/085739 A1 (FIGS. 8, 9 and 10) discloses a pipe-laying vessel for S-laying a pipeline. The vessel includes two track tensioners for supporting the weight of the pipeline and for controlling passage of the pipeline. The vessel also comprises a fixed clamp, located at an upper end of a stinger. The vessel is capable of efficiently S-laying pipeline during routine pipe-laying operations.
When an accessory is to be added to the pipeline, the fixed clamp clamps the pipeline and supports the weight of the suspended pipeline downstream of it. The pipeline is then severed upstream of the fixed clamp. The upstream section of pipeline that has been severed from the suspended pipeline is then moved upstream by the track tensioners. An accessory is then lowered into position at the top end of the suspended pipeline (i.e. the end of the pipeline on the upstream side of the fixed clamp). The accessory is then welded to the top end of the suspended pipeline. The severed pipeline is then moved downstream by the track tensioners to meet the accessory and the accessory is welded to the downstream end of the severed pipeline. The fixed clamp is then released. The pipeline (with the accessory) can then be laid as normal by controlling its passage with the track tensioners.
The vessel described in WO 2006/085739 can also be used to abandon a pipeline. In some cases, this may need to be done where there is a bad weather condition or because the laying operation is completed. In these cases, an end terminal (or abandonment head) is welded to the end of the suspended pipeline in a similar way to the accessory above. The end terminal is then attached to an abandonment and recovery winch using a cable. The fixed clamp is then released. The pipeline and end terminal are then lowered to the seabed using the winch.
However, in cases where the pipeline is damaged and water has entered the pipeline, the tension to be accommodated may be unusually high and the tensioners and/or the abandonment and recovery winch may not be able to support the additional weight of the water-filled pipeline, unless they are specifically designed to do so. That may require, a larger number of track tensioners to be provided, or a larger capacity winch to be provided.
WO 01/48410 A1 discloses a method of abandoning a pipeline using an abandonment and recovery system. The suspended pipeline is initially held by a pipe-laying and tensioning arrangement. Then a series of light, elongate members including one or more buoyant members are attached to the end of the suspended pipeline. The elongate members are then lowered into the sea using a winch and cable. At least one of the elongate members is pivotable with respect to the pipeline. At least one of the pivotable elongate members is buoyant so that the end of the elongate member at the end of the pipeline projects upwardly from the seabed. The elongate members can then be disconnected from the winch cable by remote actuation of a locking mechanism.
The present invention aims to improve on the methods of pipe-laying described above. In particular, the present invention aims to facilitate the abandonment of a pipeline that has been damaged and/or facilitate the adding of an accessory or end terminal to the pipeline.